A Diesel engine is conventionally equipped with an after-treatment system that comprises an exhaust pipe, for leading the exhaust gas from the engine to the environment, and a plurality of after-treatment devices located in the exhaust pipe, for degrading and/or removing pollutants from the exhaust gas before discharging it into the environment.
In greater details, a conventional after-treatment system generally comprises a Diesel Oxidation Catalyst (DOC), for oxidizing hydrocarbon (HC) and carbon monoxides (CO) into carbon dioxide (CO2) and water (H2O), and a Diesel Particulate Filter (DPF), located in the exhaust pipe downstream the DOC, for removing diesel particulate matter or soot from the exhaust gas. In order to reduce NOx emissions, most after-treatment systems further comprise a Selective Catalytic Reduction (SCR) catalyst, which is located in the exhaust pipe downstream the DPF.
The SCR catalyst is a catalytic device wherein the nitrogen oxides (NOx) contained in the exhaust gas are converted into diatomic nitrogen (N2) and water (H2O), with the aid of a gaseous reducing agent, typically ammonia (NH3), which is stored inside the catalyst. Ammonia is obtained through thermo-hydrolysis of a Diesel Exhaust Fluid (DEF), typically urea (CH4N2O), which is injected into the exhaust pipe through a dedicated injector located between the DPF and the SCR catalyst.
The injection of DEF is managed by an engine control unit (ECU) that calculates the quantity of DEF to be injected in the exhaust pipe, in order to achieve an adequate NOx conversion rate inside the SCR catalyst, and then commands the injector accordingly. Usually the DEF quantity to be injected is determined on the basis of the level of NOx stored inside the SCR catalyst. This parameter is measured by means of a NOx sensor placed upstream of the SCR catalyst.
A problem with this approach is that a dedicated NOx sensor upstream of the SCR catalyst is needed. Removal of said dedicated NOx sensor would be desirable while still having a reliable NOx concentration value upstream of the SCR catalyst to use for the DEF quantity determination.
At least one object is therefore to provide for a reliable estimation of the value of NOx concentration upstream the SCR catalyst without using a dedicated sensor. A further object of an embodiment is to provide for an accurate estimation of the value of NOx concentration upstream the SCR catalyst that has a high reliability over time. Another object is to provide for an estimation of the value of NOx concentration upstream the SCR catalyst without using complex devices and by taking advantage from the computational capabilities of the Electronic Control Unit (ECU) of the vehicle. In addition, other objects, desirable features and characteristics will become apparent from the subsequent summary and detailed description, and the appended claims, taken in conjunction with the accompanying drawings and this background.